Video cassette recorders reproduce the video signal on a magnetic tape by demodulating an FM luminance signal to produce the luminance or black and white portion of the composite video signal. The VCR also converts the down converted phase rotated chroma signal on the tape to a chroma information signal at a carrier frequency of 3.58 MHz. The thusly converted luminance and chroma signals are subsequently combined and applied to the video signal input terminals of a television receiver for observation of a video pattern on the screen of the television receiver.
The chroma conversion circuits in a present day VCR employ an internally developed 30 Hz chroma reference signal which ensures that the direction of phase rotation is executed correctly to produce an accurate representation of the information recorded on the tape. The phase rotation may differ depending on the type of format used in the VCR under test. For example, in the VHS format the chroma carrier is shifted in a positive direction 90 degrees for every horizontal line. This phase shift or rotation progresses in the following order for one-half of the 30 Hz pulse: 0, 90, 180, 270, 0. For the other half of the 30 Hz pulse the phase rotation is 0, 270, 180, 90, 0. Its purpose is to minimize crosstalk between different tracks on the tape.
Moreover, the luminance FM demodulator changes the frequency and deviation of the FM carrier in accordance with the particular protocol of the system used by the VCR.
One known method of troubleshooting the chroma circuits of a VCR is to use an oscilloscope to detect defects in the chroma circuits. This signal tracing method works well in some applications, but in some cases the signals being analyzed in the chroma conversion circuits are very complex and difficult to comprehend when viewed on an oscilloscope. One signal which is particularly difficult to scrutinize on an oscilloscope is the phase-rotated down converted chroma carrier which is constantly being phase rotated at the video horizontal rate.
In another method of troubleshooting the chroma conversion circuits in a video conversion circuit, an FM luminance signal and a down converted, phase rotated chroma signal are injected into the VCR circuits ahead of the FM demodulator and the chroma convertor circuits, and a television receiver is used to monitor the picture corresponding to the injected FM luminance and phase rotated chroma signals. It is known to inject other signals into the VCR circuits to detect defective circuitry therein. Such injected signals have included a 3.58 MHz reference pulse, a horizontal key pulse, and a 30 Hz reference pulse signal. All of these signals have been synchronized to the injected FM luminance and down converted, phase rotated chroma signals.
It is also known to overdrive the 30 Hz reference signal in a VCR with a separately generated 30 Hz reference signal which is in sync with the injected signals. A problem with this method is the fact that the overdriving signal is very level and phase sensitive. Also, there is potential damage to the low impedance 30 Hz drive circuits in the VCR. For example, the 30 Hz reference signal in the VCR might be running at a slightly different frequency from the 30 Hz reference injected signal. The injected chroma signal will then fall out of phase with the internally generated 30 Hz reference signal causing the chroma conversion circuits to be rotating in one direction while the injected phase rotated chroma signal is rotating in the opposite direction. When the picture is viewed on the screen of the associated television receiver, such a condition produces undesirable results in the form of bursts of color which occur when the two 30 Hz reference signals temporarily fall into phase. As a result of this condition, the service technician may be led to believe that the color conversion circuit is defective even though it might be operating properly.